Heavy water moderated organic cooled nuclear fission reactor



April 1965 R. J. RICKERT ETAL 3,180,801

HEAVY WATER MODERATED ORGANIC COOLED NUCLEAR FISSION REACTOR Filed Sept.26, 1963 4 Sheets-Sheet l j INVENTORS ROYCE J. RICKERT WILLIAM s. FLINNJOHN F. ROHLIN JOHN J. ROTH BWM:

April 1965 R. J. RICKERT ETAL 3,180,801

HEAVY WATER MODERATED ORGANIC COOLED NUCLEAR FISSION REACTOR Filed Sept.26, 1963 4 Sheets-Sheet 2,

r-CENTER REGION j (FIRST PASS) ON OUTER REGI SECOND PASS) RETURN HEADEROUTLET I INLET HEADER C HEADER 72 I 48 TO I MPS 1 I FROM STEAM PU 26 lGENERATOR OUTLET INLET HEADER C HEADER RETURN HEADER INVENTORS ROYCE J.RICKERT WILLIAM S. FLINN JOHN F. ROHLIN JOHN J. ROTH 1 27, 1965 R. J.RICKERT ETAL 3,180,801

HEAVY WATER MODERATED ORGANIC COOLED NUCLEAR FISSION REACTOR Filed Sept.26, 1963 4 Sheets-Sheet 5 @Q 3. Sm Bu on m TN l m2 mm mmw N smm U M NQ m@9 mumm I m S mwmw April 27, 1965 R. J. RICKERT ETAL HEAVY WATERMODERATED ORGANIC COOLED NUCLEAR FISSION REACTOR Filed Sept. 26, 1963LZOI FIG. 5

4 Sheets-Sheet 4 FIG. 6

INVENTORS ROYCE J. RICKERT WILLIAM S. FLINN JOHN F. ROHLIN JOHN J. ROTHU ited Sta es atent The present invention relates to a heavy watermoderated, organic liquid cooled nuclear fission reactor having 7 areduced reactivity requirement.

During recent years considerable effort has been devoted to thedevelopment of new reactor concepts which will lead to power sources atreduced costs and will co pete successfully with power plants utilizingfossil fuels. While considerable progress has been made in thisdirection, nuclear power has yet to compete successfully with alternatetypes of power generators except in special locations or under uniqueconditions, as for example, at remote locations Where fuel deliverycosts are excessive.

In order to make further improvements in the efficiencies of nuclearreactors, considerable effort has been expended to seek outmaterialswhich are most effective when utilized in a reactor and toobtain a reactor concept which uses these materials in the mosteflicient manner possible. For example, one of the best moderatormaterials useful in a thermal reactor is heavy water (D 0) which has asmaller absorption cross-section for neutrons than almost any othermaterial. Being a liquid,

in addition, it can be readily held and circulated. However, due to itshigh vapor pressure, it has to be used at a relatively low pressure toavoid expensive high pres sure equipmenu Moreover, heavy water is Veryexpen: sive so that the nuclear reactor should be designed to require avery small inventory of this material One way of reducing the amount ofD 0 required, is to limit the function of this liquid to moderator onlyand provide another liquid having a low vapor pressure as coolant. Whilea wide variety of coolants meeting this requirement are available foruse in a nuclear reactor, there are relatively few having suitablenuclear properties which will function at adequate heat transfer levelsunder relatively low pressures and at the same time are withoutundesirable handling problems. At higher pressures it is possible toeffect heat transfer more efliciently but the gains made thereby areofiset by the higher initial and maintenance costs of high pressureequipment throughout the power plant. Liquid metal coolants, which canbe used at a low pressure, have the drawback that they impose a varietyof difficult problems which can be solved 0 only with the use ofelaborate expensive capital equip: ment and overcoming complex operatingproblems. One material, however, which has generated considerableinterest in the field for use as a coolant under relatively low pressureis that of organic liquid, as for example, terphenyl.

In addition to separating the cooling and moderating functions in thereactor,-another way of reducing the size of heavy water inventory wouldbe to reduce the reactivity losses within the reactor. To illustrate thepossible savings, 21 40% increase in the cost of heavy water inventoryis required to provide an additional one percent in reactivity of anuclear reactor of the size and type to be considered further below.Thus, in. order to provide a nuclear reactor which makes themost.efiicient use of heavy water, it is necessary to provide a reactordesign which eliminates reactivity losses to an extent heretofore neveraccomplished in nuclear reactors of the size requlr ed for commercialproduction of power."

Another feature which would be highly desirable in a.

using either natural or slightly enriched uranium fuel. Flexibility ofthis type makes it possible to step up power output without expensivereconstruction and to take advantage of favorable sources of supply and'cost advantages found in one type of the fuel.

The present invention offers a reactor concept which makes it possiblefor the first time to utilize heavy water in the most eflicient mannerever obtained in a power reactor whereby further substantial increasesin efficiency are made to produce power at a cost which will be morecompetitive with conventional power sources. In addition the inventionpermits the use of fuel which is either natural uranium or slightlyenriched without making changes in the reactor construction.

It is a well known consideration in nuclear reactor design toincorporate suflicient excess reactivity to compensate for certainexpected losses, changes in reactivity overthe life of the reactor, andinequality of neutron fluxes throughout the core in order to insure thatthere will always be, during the design life of the reactor, sufficientreactivity to maintain the desired levels of opera ation. The nature ofthe various reactivity losses are usually grouped into the followingcategories:

' (1) Parasitic neutron capture in cladding, structural metal, thecoolant;

(2) Neutrons lost to control absorbers; and

(3) Neutronslost through increased leakage caused by flattening of thegross power distribution in the core.

The neutron losses in the first group are'minimized by carefulthermal-hydraulic and mechanical design which yields the lowest powercost consistent with the nuclear requirements. Minimizing of the neutronlosses in the second and third groups, however, is a rrroredifficultproblem which has received considerable attention by workers in thefield for a long time.

In order to increase the heat output capability of a reactor, it iscommon pr'acticeto flatten the radial power distribution in the core bydiiferential loading of the fissile material in the radial directionwhich flattens the neutron flux in the central region of the core. Thispractice causes a loss in reactivity because it increases theprobability for neutron leakage. In addition, burnup of stationary fuelin the higher flux region in the center of the core can produce powerflattening with the attendant .loss in reactivity due to increasedleakage. In a heavy water reactor designed cooperate on natural uraniumfuel,.any reactivity loss can only be compensated, to obtain a criticalsystem, by adding D 0 or by reducing the design life, i.e., burnup, ofthe fuel. Both of these methods reflect heavily on the economies of theplant.

By the present invention, important reductions in neutron losses arisingfrom the use of control absorbers and neutron flux gradients throughoutthe core are accomplished to an extent not previously consideredpossible in a power nuclear reactor utilizing a low pressure coolantsuch as an organic liquid and a moderator of heavy water.

region and the. hotter, second-pass coolant in the outer.

region balances the limiting temperature conditions to 7, permit, theheat to be extracted from a fewer number of practical and efficientreactor would be the capability of channels.

Briefly described, the-'inventivereactor includes a unique;- 7arrangement in. which no excess reactivity is provided which has to becontrolled during'steady-state operation.

The design is such that the fuel can and is moved during reactoroperation to insure that the radial and axial gross flux and powerdistributions are always the same as those of a freshly loaded core.Thus, no reactivity is lost due to flattening of the central neutronflux. Another unusual feature of this reactor is a complete distributionof fuel at all burnups in each flow channel to insure that the poweroutput of each channel always remains constant. Other featurescontributing to the final result appear below.

It is thus a first object of this invention to provide a nuclear reactorin which neutron losses are substantially reduced.

Another object is an organic liquid cooled, D moderated nuclear reactorwith reduced neutron losses.

Still another object is a nuclear reactor with a core having provisionto move, remove and replace fuel during normal operation to maintain areactor without excess reactivity during its life of operation.

Other objects and advantages of this invention will hereinafter becomeobvious from the following description of a preferred embodiment of thisinvention taken with the accompanying drawings in which:

FIG. 1 is an elevation view, partially schematized to show a single rowof pressure tubes, and in section, of a reactor embodying the principlesof this invention;

FIG. 2 is a plan view of the reactor in FIG. 1;

FIG. 3 is a schematic of the core arrangement for this reactor;

FIGS. 4A, 4B, 4C and 4D show a typical pressure tube construction,partially cut away, extending the length of the reactor shown in FIG.

FIG. 5 is a typical fuel element assembly in section to show thestaggered ends of the fuel rods; and

FIG. 6 is a view along 6--6 of FIG. 5.

Referring to FIGS. 1 and 2, nuclear reactor 10 comprises a sealed,cylindrical moderator tank 12 having a lower inlet nozzle 14 and anupperoutlet nozzle 16, as well as upper and lower walls 12a and 12b,respectively. Lining and spaced from the walls of tank 12 if desired arethermal shields 18, 22, and 24. Extending completely through moderatortank 12 are a plurality of spaced, vertical coolant tubes 26, which aswill be seen later, contain the fissionable fuel of reactor 10 andthrough which the coolant flows to withdraw the heat of fission. Themoderator liquid contained within tank 12 flows around tubes 26 whichare completely sealed except for connecting headers and removable endcaps which will be described later.

Above and below moderator tank 12 are a pair of upper and lowercylindrical shield tanks 28 and 32, respectively, filled partially withsuitable radiation shielding material such as iron shot 31 and 33.Coolant tubes 26 pass completely through shield tanks 28 and 32.

The assembly of moderator tank 12, shield tanks 28 and 32, and coolanttubes 26 are supported within and spaced from a concrete wall 34 by twoor more bracket assemblies 36 for upper shield tank 28, bracketassemblies 38 for moderator tank 12, and bracket assemblies 42 for lowershield tank 32. Moderator inlet and outlet nozzles 14 and 16 would beconnected to conduits (not shown) passing through openings 44 and 46 inconcrete wall 34 to permit circulation of the moderator and cooling ofthe latter externally of reactor 10.

One of the features of this reactor design is a twopass, bi-directionalcoolant flow arrangement to minimize the influence of the non-uniformneutron flux intensities across the core in determining the limitingthermal conditions and, thus, to permit a higher average heat output fora fixed number of channels. In this construction, the centrally locatedcoolant tubes receive the first pass of coolant. Return headers areprovided to mix the first pass fluid so as to supply a uniformtemperature coolant to the second pass coolant tubes. to theirrespective headers at opposite ends so that these Adjacent tubes areconnected.

tubes always have flow in opposite directions. In this way there is astrong tendency to provide uniform temperatures throughout the coreregion.

Returning to FIGS. 1 and 2 for a more detailed description of thisarrangement and to FIG. 3 for a schematic view of the reactor, it isseen that reactor 10 is provided with a pair of semi-annular inletheaders 48 and 50, respectively, partially surrounding the upper andlower regions of moderator tank'12. Coolant is supplied from a commonpipe or conduit 51 to the upper region of tank 12 through header 48,sub-headers 84 and jumper tubes 53 to the upper portion of first passcoolant tubes 26. Fresh coolant supplied to the lower region of tank 12is supplied by way of sub-headers and jumpers (not shown). As pointedout earlier, the coolant tubes 26 in the central region carry the firstpass of coolant while those in the outer periphery carry the second passof the coolant, and as shown schematically in FIG. 3, adjacent tubes 26receive coolant from upper and lower main headers 48 and 50,respectively, so that flow is in opposite directions in adjoining tubes.The upper and lower downstream ends of first pass tubes 26 are connectedby jumper tubes 54 and 56 respectively, to sub-headers 82 and 86 andmain headers 58 and 62, respectively, where'the coolant from the variouschannels are mixed to obtain a uniform temperature. Main headers 58 and62 are connected by way of sub-headers 87 and 88, and jumper tubes 64and 66, respectively, to opposite ends of adjoining tubes 26 in theouter periphery for the second pass of coolant. Downstream of the Isecond pass of tubes 26 the coolant is led to outlet main headers 72 and74, by way of sub-headers 89 and 90 where the flows are combined and letout through a common pipe or conduit 76 for use in generating the steamoutput of the power plant incorporating this reactor.

Another important feature of this nuclear reactor is the arrangement ofthe fuel within tubes 26 to permit onstream refueling and a shuflling offuel during operation of the reactor to maintain a complete distributionof fuel at all burnups within each tube 26 to assure that the radial andaxial gross flux and power distributions remain substantially constant,so that no reactivity is lost due to flattening of the central neutronflux.

For the details of a typical coolant tube 26a reference is made to FIGS.4A, 4B, 4C and 4D which illustrate tube 26a in portions. It will benoted that tube 26a consists of an upper section 102, a central sectionor calandria tube 104, and a lower section 106. Upper section 102receives its coolant from a jumper tube 53a and is sealed off at the topwith a cap 108 which is threaded into engagement with thimble opening112 of tube 26a. Lower section 106 has a jumper. tube 56a for coolantleaving tube 26a. Seal cap 108 is provided with a shaft 114, pin 116,and holes 117 used in removing cap 108 when desired. Upper section 102terminates, as seen in FIG. 4B, in top wall 12a and shield 24 ofmoderator tank 12.

Within tube 26a is suspended an inlet guide tube 118 from a threadedflange 122 spaced from the outer wall of 26a and held in proper spacedrelationship by spacer members 124. Guide tube 118 is provided with anumber of large holes 126 as illustrated to permit the coolant to flowfreely around and through it.

From FIG. 4B it will be seen that at the lower end of guide tube 118 aninner pressure tube 128 is suspended from a flange 132 threaded intoupper section 102. Close-- ly spaced from and surrounding inner pressuretube 128, beginning at a point just below the upper wall of moderatortank 12 is a calandria tube 104 attached to top shield 24 by anyconvenient means, such as by welding, for passage through moderator tank12. The space between pressure tube 128 and calandria tube 104 ispressurized? by an inert gas by way of pipes 136 and 138 and a sealarrangement 139 to prevent'leakage of moderator into the coolant, andvice versa. In similar fashion calandria coolant tube 26a. A threadedseall'cap 156 with a rod 158 and a pin 16}; for engagement provideclosing and sealing of coolant tphe 26a at thebott om.

Within tube 26a extending between upper and lower seal caps 108 and 156is located a solid's'pacer or fuel positioning rod.164 having members166 and 168 to lock with guide tubes 118 and 151. A pair of pins 172 and174 facilitate'attachment when removed or replaced. Rod 164support s bycontact only several fuel assemblies 176 within moderator tank 12, andhas spacer members 178 at various points along the length thereof toinsure proper centering of rod 164 atall times. By the arrangementillustrated and just described, it will be seen that fuel assemblies 176can be pushed out either end of tube 26c by removing end caps 108 and156 and pulling and pushing rod 164 (jut.

The details of a typical fuel assembly 17 6 are shown in FI GS LS and 6.Fuel assembly 176 is composed of a cluster of fuelrods 192 which are oftwo diameters, 192' and "1 92', which are-grouped together in a circularpat-' tern and held together'by'bands 201. Five of the assemblies 2176are aiially girouped'within pressure tube 128 as illustrated in FIGS.4B'-and 4Q, The ends of fuel rods 1921 ar e staggered as shown FIG. 5 soas net to concentrate the loss of fuel dueto end caps in one smallregion at the end of each fuel assembly; This reduces flux peaking, Theouter ring of fuel elements 192, however, are of'the samelength so thatthe assemblies grouped together will have equal and balanced bearingsurfaces, as illustratedinl- IQSQAB and 4C.

Each fuel rod"192 is composed of U0 sheathed in a tube 202"'on which anintegral spiral-key 204 has been extruded. The ends of "tubes'- 202 areclosed off with end caps 205. Key 204 acts as a spacer between individualfue'l rods 192 when-assemblyllo is banded together. It will he notedthat .the'fuel assemblies 176 merely abut each other without positiveconnection as is also the case with rQd164. an h nd. fu l asemblies ecomp et abse ce f coup n n. e r edu e parasit c n utron ca ture, Th u oing W 5 1 merely act as spacers for maintaining the proper positionoffuel. assembliesand also make possible the removal of the latter aswill be seen below. i

While the 'arragnement forthe control of this reactor ha ot been esc b di is nd rst od ha on rol, r can be inserted into the moderator betweenthe coolant tubes regulated in a way now well understood in the art. Analternative arrangement for the control of this reactor is the so-calledHy Ball type in which columns of b orat ed steel balls are inserted inmoderator tank 12. By use of hydraulic means the balls are pumped in andout of themoderator region of the reactor. While this type of poisoncontrol is listed in the table below, T isunder stood'that eithertype,nei ther being a part of this inventiomcanbeutilized. i

Intheoperation of the reactor described, it is seen that refueling takesplace without shutting the power plantdownf'At regular intervals, bothends ofa cool ant tube26 are opened by removal of the seal caps. =Tl1'espacer bar 16d is then usedto push fnel assemblies-176 in the direction(if coolant flowuntil; one assembly, typically, is removed from thedownstream end. A freshfuel asembly is then inserted intotheupstreamlend of the coolant tube by manipulating spacer rod 164. Hjence,

it is seen that the hottestcoolantis at the end of the channel incontact with the fuelgenerating the lowest 6 POW?! in the channe to pr de m r m ie t t e m e i n- In fact, it will be seen that over the wholelife of the reactor, after an initial period of adjustment, each channelwill have throughoutits length and entire life of service substantiallyconstant fuel composition so that the radial and axial flux and powerdistribution will not vary in time. Thus, -no reactivity is lost due toflattening of t e t a neutron flu s o urs i ea o s o conventionaldesign.

This method of fuel management results in the least reactivityrequirement to provide a burnup of 5000 mwd./ t. with natural uraniumfor the reactor described in the table below. Hence, the nuclearadvantage of minimum r a y requ e e wi h this r ue n m th is tainedwithout levying a large penalty on the thermalhydraulic esig 7 A furtheradvantage of the refueling method is apparent when the easeor fuelburnup to 20,900 mwd./t. with enriched fuel is considered. Fuel which isready to be discharged from the reactor has experienced a largereduction in total fission cross section as compared to freshly loadedfuel. Since each flow channel contains a complete distribution of fuelat all burnups, the power output of the channel always remains constant.Therefore, the thermal design does not have to provide for widelyvarying f'power butputs per channel between the natural uranium-and theenriched uranium case. As the fuel proceeds down the channel, theincreasing burnup eventually reduces the local power generation in theelement. Furthermore, the continuous refueling of the-reactor withoutshutdown eliminates t-he needior downtime found other reactors andcontributes materially to the efiiciency of the reactor as a powersource. A

Parameters of a reactor design for a 5.00 mwe. size unit (with bothnatural and slightly enriched fuel) in accordance with. this invention,are given in the table.

It is thus seen that there has been provided a nuclear reactor-utilizingD 0 moderator more .efliciently than be-' 7 Table General: 1

Reactor type; D10 moderate, organic p 7 cooled. Steam cycle typeNon-reheat with turbine moisture extraction. Total fission power,including mpdera- 1,550.

tor heat loss, mw.

Radial reflector thicknes 'Axial reflector thickness,

Turbine gross electrical ontput, mwe 542.3. Station netelectricaloutput', mwe Enriched, 512.2.

Natural, 510.3. Net plant heat rate, B.t.n ./kw -hr Enriched, 10,330.

Natural, 10,370. Net plant efiieiency, percent n Enriched, 33.05.

Natural, 32.92. Fuel and core:

Type of fuel U02. Fuel density, percent oft 91. Design burnup, mwdJmEnriched, 20,000.

' Natural, 5,000. Cladding material..- XAP+00L Cladding thickness, i0.010 and 0.016. Fuel rod O.D., in 0.313 and 0.513. Number of fuel rodse 6 plus 31. f Overall length 01 fuelhun '40. i Number of bundles perpress ng tub 5. Coolant tubes: V V

600. Lattice arrangement Square.

it h 9%. Material Zr"-2 and XAP-OOI. Outside diameter, in 4%2'. Wallthickness, in Zr-2 0.036 aud-XAP 0.062.

Insulation materia Core length, in Core equivalent diameter 1 Total U01loading, lbs" Type of control unit... Number of control units r 0.100in. inert gas annulus.

Nuclear:

Primary coolant system:

Moderator System:

Secondary system:

TableContinued Enriched Reactivity allowances, percent:

Cold to hot, moderator to 200 F coolant to 650 F- Zero to full power(Doppler) Clean to equilibrium xenon,

Samarium.

Burnup (first core peak to equilibrium Maximum excess reactivity,percent Design life, mwd./metric ton of U Fuel conversion ratios, byweight:

Gross lifetime:

Total Pu generated/U-235 lost. Fisslonable Pu generated/U- 235 lost.Nlet lifetime, Pu remaining/U-235 os Lifetime energy production,percent:

U-235 fission U-238 fast fission Pu fission Thermal:

Average heat flux, B.t.n./hr. i't. Maximum heat flux, B.t.u./hr. ft.Flux or heat generation ratios, peak/ averagez.

Basic radial Basic axial- Radial local Axial lnnal Maiijimum cladsurface temperature,

Maximum U0 fuel temperature including all hotspot factor, F Minimum D NBCoolant material Reactor pressure drop (header to header), p.s.i.

Number of loops Type of pumps.

Number of pumps per loop Coolant pressure at pump intake,

p.s.1.a. Pump discharge pressure, p.s.i.a

Pumping power per pump, kw(e)..

Coolant inventory, lb

ket gas Materials in contact with coolant e rum Moderator tank inlettemperature, F- MgxFrator tank outlet temperature,

Design temperature, F Total flow rate, lb./hr

Type of pumps- Pump head at rated flow, p.s.1 Pumping power per pump, kwSystem design pressure, p.s.i.g- Blanket gas Materials in contact withmoderator..- Total inventory, lb- Makeup, lb./year Coolingarrangementnns e rum Steam generator inlet temperature,

9 F. 7 Steam generator outlet temperature,

Superheater inlet temperature, F Superheater exit temperature F Steamgenerator pressure, p.s.1.a. Superheater exit pressure, p.s.i.a Flowrate thru steam generator, lb./hr- Steam flow to turbine, lb./h!

ratio Enriched, 223. Natural, 259.

4. ilentrlfugal.

Enriched, 347.

Natural. 383.

Enriched, 3,600.

Natur Nitrogen.

Carbon steel, XAP-001, stainless steel. I

1. Centrifugal. 50.

elium. Stainless steel, zircaloy. 519,000.

5,000. Non-regenerative.

What is claimed is:

1. A nuclear fission reactor comprising a sealed moderator tank, spacedcoolant tubes having end assemblies extending completely through saidtank, fissionable fuel assemblies within each of said coolant tubesforming the reactive core of said reactor, means for permitting thereplacement of fuel at regular intervals during normal operation of saidreactor, the latter said means including spacer means for maintainingsaid assemblies in predetermined axial positions within said coolanttubes, said fuel assemblies in each coolant tube being in axial arrayand without positive interconnection, said spacer means aligned axiallywith the fuel assemblies in each coolant tube similarly in surfacecontact with the end assemblies to permit the selective removal andreplacement of fuel assemblies from each fuel coolanttube by withdrawingand pushing said spacer means through each said tube, means for pumpingheavy water at relatively low pres sure through said moderator tank andaround said coolant tubes to moderate the neutrons produced as theresult of the fission of said fissionable fuel in said fuel assemblies,and means for circulating through said coolant tubes in contact withsaid fuel assemblies at a relatively low pressure an inert liquidcoolant to remove the heat of fission of said reactor.

2. The nuclear reactor of claim 1 in which means are provided to supplysaid coolant to each of said coolant tubes adjacent one end thereof andremove same adjacent the opposite end thereof such that flow throughadja'cent tubes is in opposite directions and in the direction of fuelmovement during removal and replacement of fuel assemblies, thereby saidfuel assemblies in each said coolant tube being always arranged so thatthe freshest fuel is at the upstream end of each said coolant tube andeach subsequent fuel assembly in the downstream directilon is moredepleted than its adjacent upstream assemb y. r

3. The nuclear reactor of claim 2 in which means are provided tocirculate said coolant in two complete passes through said tubes, thefirst pass being through coolant tubes located in the central region ofsaid core and the second pass of said coolant being through theremaining tubes located in the outer region of said core thereby tendingto render the temperature through said core more uniform.

I 4. The nuclear reactor of claim 3 in which a first pair ofsemi-annular main headers are disposed adjacent and around the upper andlower portions of said moderator tank to receive fresh coolant, means todistribute said coolant in said headers to said coolant tubes, a pair ofannular headers adjacent the upper and lower ends of said tubes, meansfor distributing coolant at the end of the first pass to said annularheaders for mixing, means for distributing the mixed coolant into thesecond pass coolant tubes, and a second pair of semi-annular mainheaders oppositely facing said first pair of semi-annular headerstoreceive heated coolant completing the second pass through said reactor,and means for removing the heated coolant for external utilization.

(References on following page) 1 0 References Cited by the Examiner3,093,565 6/63 Blockley et a1. 176-59 UNITED STATES PATENTS 3,108,05310/63 Vrillon 61; a1. 1765 8 g; g i 52-23 FOREIGN PATENTS m ermyer I 1259 Liljeblad 5 5 1,162,270 4/ 58 Frame- Evans et aL France- 5 1 Rand 1731 1,297,266 5/62 France.

2/63 Bell 176- 61 1,299,368 6/62 France.

3/63 Dickinson 176-61 4 /53 Tunnich'fle 17 30 1O CARL D. QUARFORTH,Przmary Exammer.

1. A NUCLEAR FISSION REACTOR COMPRISING A SEALED MODERATOR TANK, SPACEDCOOLANT TUBES HAVING END ASSEMBLIES EXTENDING COMPLETELY THROUGH SAIDTANK, FISSIONABLE FUEL ASSEMBLIES WITHIN EACH OF SAID COOLANT TUBESFORMING THE REACTIVE CORE OF SAID REACTOR, MEANS FOR PERMITTING THEREPLACEMENT OF FUEL AT REGULAR INTERVALS DURING NORMAL OPERATION OF SAIDREACTOR, THE LATTER SAID MEANS INCLUDING SPACER MEANS FOR MAINTAININGSAID ASSEMBLIES IN PREDETERMINED AXIAL POSITIONS WITHIN SAID COOLANTTUBES, SAID FUEL ASSEMBLIES IN EACH COOLANT TUBE BEING IN AXIAL ARRAYAND WITHOUT POSITIVE INTERCONNECTION, SAID SPACER MEANS ALIGNED AXIALLYWITH THE FUEL ASSEMBLIES IN EACH COOLANT TUBE SIMILARLY IN SURFACECONTACT WITH THE END ASSEMBLIES TO PERMIT THE SELECTIVE REMOVAL ANDREPLACEMENT OF FUEL ASSEMBLIES FROM EACH FUEL COOLANT TUBE BYWITHDRAWING AND PUSHING SAID SPACER MEANS THROUGH EACH SAID TUBE, MEANSFOR PUMPING HEAVY WATER AT RELATIVELY LOW PRESSURE THROUGH SAIDMODERATOR TANK AND AROUND AID COOLANT TUBES TO MODERATE THE NEUTRONSPRODUCED AS THE RESULT OF THE FISSION OF SAID FISSIONABLE FUEL IN SAIDFUEL ASSEMBLIES, AND MEANS FOR CIRCULATING THROUGH SAID COOLANT TUBES INCONTACT WITH SAID FUEL ASSEMBLIES AT A RELATIVELY LOW PRESSURE AN INERTLIQUID COOLANT TO REMOVE THE HEAT OF FISSION OF SAID REACTOR.